R2 signalling is a form of inter-switching office signalling which has been standardized by CCITT. This type of signalling uses dual tone multi-frequency (MF) signals, and is referred to as "compelled". In R2 signalling, an outgoing register of an originating switching office sends a first forward MF inter-register signal which is detected and recognized at a receiving office, which can be at the far end of a sequence of tandem switching offices. Upon recognizing the first forward inter-register signal, the receiving office sends a backward inter-register MF signal to the originating office, which has its own meaning and at the same time serves as an acknowledgment. This backward inter-register signal is detected and recognized at the originating office whereupon it sends the next forward inter-register signal to the receiving office. Again the receiving office sends a next backward inter-register signal to the originating office, and the back and forth communication continues until the last inter-register signal has been sent. The communication between switching offices is conducted between an outgoing register of the originating central office, via a trunk, to a similar incoming register at the receiving central office.
The system of communication is referred to as "compelled" because the originating switching office transmits signals in response to backward signals provided by the receiving switching office. The compelled nature of the protocol gives the R2 form of signalling significant flexibility. This includes not only the ability to transmit a variety of information, such as address signals, congestion signals, calling and called party status signals, etc., but also the ability to work end-to-end. End-to-end signalling is the ability to communicate directly with a far end incoming register, even if several intervening central offices have been used to route the call.
While the CCITT standard is usually used between international switching exchanges, numerous variants have been created for national use. Some of those variants have little in common with the CCITT R2 protocol except for the signalling frequencies used.
The fact that various variants of the standard have been implemented by various countries has resulted in local national switching offices that must be provided uniquely programmed to implement the local national protocol. Since most modern switching offices are program controlled, should changes be required to the switching office due to a change in national protocol or due to the switch being moved, etc., new switching office computer programs are required to be resident in the switching office. Typically, these programs are supplied on EPROM, floppy disk or magnetic tape and are supplied in each instance by the switching system supplier which is totally familiar with the programming requirements of its switching system product.
Changing or updating of a switching office due to a change in the inter-office communications protocol requires, however, significant programming effort on at least a country by country and switching system by switching system basis, which is costly to the customer and time consuming for the supplier.
In addition, with the expansion of world trade in switching systems, it is desirable for a switching system supplier to be able to provide a single switching system that can be easily adapted to provide and receive a large variety of inter-office signalling protocols or all variants of a standard signalling protocol at minimum cost. Until the present invention, this has not been achievable at a significantly low cost and with as much ease as is possible using the present invention.